1. Field of the Invention
The present invention relates to an apparatus and method for monitoring, controlling, displaying and dissipating an electrostatic charge from an object, such as a computer monitor, television, telephone handset or workbench.
2. Description of the Related Art
There are several problems created by the accumulation of electrostatic charge on an object such as damage to sensitive electrical devices that occurs during an electrostatic discharge event. An electrostatic charge is typically generated by people in two ways, triboelectrification and induction. Triboelectrification is caused by contact and separation between two similar or dissimilar materials. For example, when a person walks across the room, each shoe repeatedly comes in and out of contact with the floor surface, such as carpet. This causes the generation and accumulation of electrostatic charge on the person through triboelectrification. Triboelectrification also occurs in devices. For example, a belt being pulled over a pulley is constantly coming in and out of contact with the pulley which generates electrostatic charge through triboelectrification.
Another way an electrostatic charge is generated or accumulated on a person is by induction. By induction, an electrostatic charge on one object is induced to another object having a lower electrostatic potential. For example, a conveyor belt running near a person would accumulate an electrostatic charge. This electrostatic charge is induced to the person near the belt where that person is typically at a lower electrostatic potential than the charge on the belt. Electrostatic induction similarly occurs to a person sitting near a cathode ray tube ("CRT") of a computer monitor or television screen. The CRT is constantly generating a large amount of positive charge, while the person near the CRT typically has a negative charge. Therefore, an electrostatic charge is induced from the CRT to the person.
The above two methods are the most common methods that electrostatic charge is induced on an object or person. These two methods of generating electrostatic charge lead to at least three failure modes in electronic equipment: catastrophic electrostatic failure, latent electrostatic failure, and irradiated electrostatic failure.
A catastrophic electrostatic failure occurs where the electrostatic charge of an electrostatic event destroys the non-conductive oxide layer of integrated circuits, surface mount resisters, and other sensitive electronic components. Catastrophic electrostatic failure usually results in immediate, irreparable damage to the electrical component.
Latent electrostatic failure is probably the most common failure mode caused by electrostatic charge. In latent electrostatic failure, accumulated electrostatic charge weakens the non-conductive oxide layer of sensitive electronic components. This weakening of an electronic component leads to intermittent problems where sometimes the component works and sometimes the component does not work. Such intermittent problems are probably the most difficult problems to trace.
Irradiated electrostatic failure is probably the least common failure mode. In an irradiated electrostatic failure, an electrostatic event occurs near a sensitive electronic component. The sensitive electronic component, such as a microprocessor, interprets the electrostatic event as a command which ultimately causes a system glitch. Irradiated electrostatic failure can also corrupt software stored on electromagnetic media such as floppy disks or hard disks.
With telephone handsets and headsets, the electrostatic problem typically arises from triboelectrification where electrostatic charge is generated by the person using the handset/headset walking or pacing. With tele-marketers, the electrostatic charge is typically induced into the user of the handset/headset when the user touches or is near a computer screen. The electrostatic charge causes a distinct crackle sound in the receiver of the handset/headset. A painful electrostatic event may also occur between the user's ear and the handset/headset. With repeated accumulation and discharge of electrostatic charge, the handset/headset is eventually damaged and must be replaced.
A number of electrostatic dissipaters have been developed to address the problems created by electrostatic charge. For example, Cooter, et al., U.S. Pat. No. 5,406,443, concerns a static electricity dissipation system for computers. Alm, U.S. Pat. No. 5,357,396, concerns an earth discharge carrier intended to dampen and discharge electrostatic fields from a monitor or keyboard. Wescott, et al., U.S. Pat. No. 5,450,277, concerns an electrostatic discharge device to discharge electrostatic energy from a computer operator or optical element such as a filter mounted on a CRT computer monitor.
However, these and other conventional techniques for dissipating an electrostatic charge are themselves susceptible to damage from the electrostatic charge. For example, through time, repeated conduction by a resistor of an electrostatic charge, and particularly the initial electrostatic event, deteriorates the performance of the resistor, whether the resistor is part of the object to be protected or part of the electrostatic dissipater itself. This is because the electrostatic charge weakens and eventually destroys the non-conductive oxide layers of a resistor which are interposed between the conductive layers of the resistor. Indeed, the non-conductive oxide layers of surface mount resistors are relatively thin and particular susceptible to an electrostatic charge and, hence, surface mount resistors are not conventionally used to miniaturize electrostatic dissipating devices.
Further, means for displaying the electrostatic charge being dissipated were limited because the electrostatic charge, and particularly the initial electrostatic event, would damage the display device. For example, repeated exposure to electrostatic charge causes a liquid crystal display ("LCD") to have darkened areas as the LCD is gradually damaged by electrostatic charge.